The present invention relates generally to integrated circuit fabrication, and more particularly to a coater for dispensing photoresist onto a substrate.
An initial step in the photolithographic fabrication of an integrated circuit is to coat a substrate, such as a silicon wafer, with a layer of photoresist. In one process, generally known as "spin-on" coating, the substrate is positioned on a rotating chuck and photoresist is dispensed onto the center of the substrate as it rotates. The centrifugal force generated by the rotation of the substrate urges the photoresist radially outward, in the process covering the substrate surface. Once the substrate has been coated, it may be removed from the coater and baked to cure the photoresist. Finally, the cured photoresist is exposed and chemically developed, and the substrate is etched to define circuitry features.
One problem encountered in photoresist coating is non-uniformity of the photoresist layer thickness. For example, during etching, the thicker portions of the photoresist layer may be underexposed whereas the thinner portions may be overexposed, resulting in defects and, thus, a decrease in process yield. One source of this non-uniformity is fluctuation in the droplet size and flow rate from the photoresist dispenser. Premature curing, in which the photoresist cures before it reaches the substrate edge, also produces a non-uniform surface.
Another problem is that some conventional photoresist coaters are inefficient and waste significant amounts of photoresist. As much as ninety-seven percent of the photoresist may be spun off the substrate surface during the coating process. There may also be additional costs in properly disposing of the photoresist if it is considered a hazardous waste.